At present, oxide thin film transistors are mainly zinc oxide based thin film field effect transistors of polycrystalline oxide semiconductor materials, while non-crystalline oxide semiconductors such as oxide thin film transistor adopting indium-gallium-zinc oxide (IGZO) semiconductor as the semiconductor layer, have been paid much attention to in the liquid crystal industry due to their advantageous properties such as high mobility, low subthreshold value, low current and low temperature fabrication process. However, there are still some problems to be solved with regard to the existing non-crystalline oxide semiconductor thin film transistors. For example, it is difficult to form good electrical contact between source and drain electrodes formed of metal material and semiconductor layer of non-crystalline oxide.
Oxide thin film transistors may be applied to X ray flat detection devices. A sectional structure of an existing oxide thin film transistor includes as shown in FIG. 1:
a gate electrode 902 formed on a substrate 901;
a gate insulating layer 903 formed on the gate electrode 902 and covering the entire substrate 901;
a semiconductor layer 905 formed on the gate insulating layer 903;
an ohmic contact layer 906 formed on the semiconductor layer 905;
source and drain electrodes 907 formed on the ohmic contact layer 906;
a signal terminal (PIN), formed on the drain 907, consisting of a P-type layer 908, an I-type layer 909 and a N-type layer 910;
a conducting layer 911 formed on the N-type layer 910;
a first passivation layer 912 covering the source and drain electrodes 907, the semiconductor layer 905 and the conducting layer 911;
a first electrode 913 disposed on the first passivation layer 912 and connected with the conducting layer 911;
a second electrode 915 disposed on the second passivation layer 914 and the first passivation layer 912 and connected with the source electrode,
In the existing oxide thin film transistor, the signal terminal is a photodiode with PIN structure. By applying a voltage to the conducting layer 911, the first electrode 913 connected with the conducting layer 911 has a voltage, and liquid crystal molecules above the oxide thin film transistor rotate under the control of the electric field. A photosensitive layer (not shown in FIG. 1) is further provided correspondingly on the PIN. Incident light hits the photosensitive layer after passing through the liquid crystal molecule layer. Then, the PIN is in reverse conducting due to a voltage difference existing between the conducting layer 911 over the PIN and the underlying drain electrode 907b, which converts a light signal into an electrical signal and transmits it to the drain electrode 907b. 
At the same time, a voltage is applied to the gate 902. Therefore, the electrical signal passes from the drain electrode 907b through the ohmic contact layer 906 and the semiconductor layer 905 and arrives at the source electrode 907a. The second electrode 915 connected with the source electrode 907a obtains the signal. The second electrode 915 is further connected with a detection circuit of the flat detection device and therefore can detect intensity information of the X ray for example.
The above-mentioned source and drain electrodes 907 is generally made of metal such as aluminum, molybdenum, titanium and indium oxide. The electrical characteristics of this kind of oxide thin film transistors tend to be influenced by carriers in the semiconductor layers 905, making the interfacial contact resistance between the source and drain electrodes 907 and the semiconductor layer 905 form a heterojunction or a homojunction, which greatly influences the driving current. Although the existing oxide thin film transistors adopts the ohmic contact layer 906 disposed between the source and drain electrodes 907 and the semiconductor layer 905 to decrease contact resistance, a certain resistance still exists due to the difference in valence bands of the metal and the semiconductor.